Before being discharged into the open air, the exhaust gas may be purified by means of a filter body, such as those shown in FIGS. 1, 3 and 4, known from the prior art.
A particulate filter 1 conventionally comprises at least one generally cylindrical filter body 3 of longitudinal axis C-C, of length typically between 10 and 30 cm, which is inserted into a metal can 5.
To benefit from good thermomechanical resistance, in particular during the regeneration phases, it is advantageous to manufacture this filter body by assembling a plurality of what are called “unitary” filter blocks 11 by means of joints 12, followed by machining. The filter body is then an “assembled” filter body, as shown in FIG. 1. FIG. 2 shoals an example of a unitary filter block of axis D-D, of length “L”, of width “l” and height “h”.
The filter body 3 may also be monolithic, that is to say made as a single piece, with no joints, as shown in FIG. 3.
Conventionally, to manufacture a unitary filter block or a monolithic filter body, a ceramic (cordierite, silicon carbide, alumina, mullite, silicon nitride, a silicon/silicon carbide mixture, etc.) is extruded through an extrusion the so as to form a honeycomb preform.
To manufacture a unitary filter block 11, the extrusion the is conventionally shaped so that the lateral surface 13 of the preform has four substantially identical lateral faces 14a-d, defining for example a parallelepipedal preform of square or rectangular or even hexagonal cross section. The width “l” of a lateral face of such a preform is typically between 30 mm and 100 mm.
To manufacture a monolithic fitter body, the extrusion die is conventionally shaped so that the preform has the form of a cylinder of circular or ellipsoidal cross section.
The preform is then sintered to form a honeycomb structure.
A “honeycomb” configuration means that the preform and the porous structure comprising a set of adjacent channels 18, or “duct”, so as to form, in cross section, a checkerboard pattern.
The channels 18, each bounded by a lateral wall 22, are generally straight, of substantially square cross section, and extend parallel to one another. In one cross section, they thus form rows 19 and columns 20. The thickness of the lateral was may especially be between 180 and 500 μm. The cross section of the channels may especially be between 0.4 and 9 mm2.
Each channel of the preform emerges via an upstream opening 24e on an upstream face 26e, or “intake face”, and via a downstream opening 24s on a downstream face 26s, or “discharge face”.
The terms “upstream pattern” and “downstream pattern” refer to the images of the upstream and downstream faces, respectively, seen in a direction of observation perpendicular to these faces, in FIG. 2, the observer Oe observes the upstream pattern and the observer observes the downstream pattern.
When all the channels have the same length and the upstream and downstream faces are perpendicular to the direction of the channels at the upstream and downstream faces, and before the channels are plugged, the upstream and downstream patterns thus correspond to the cross section, that is to say perpendicular to the direction of the channels, at the upstream and downstream faces respectively.
It is conventional to distinguish interior channels 18i from peripheral channels 18p. 
Unlike the interior channels 18i, the lateral wall of the peripheral channels 18p is partly exposed to the outside of the honeycomb structure. In a honeycomb structure having longitudinal edges 29, and especially in a parallelepipedal honeycomb structure, a distinction is made, among the peripheral channels 18p, between the corner channels 18p″ and the lateral channels 18p′. The corner channels 18p″ extend along said longitudinal edges. The lateral channels 18p′ are, unlike the corner channels 18p″, positioned along only one lateral face 14a-d of the honeycomb structure.
A honeycomb structure intended for manufacturing a filter body is then alternately plugged on the upstream face 24e or on the downstream face 24s by upstream plugs 30s and downstream plugs 30e, respectively, as is well known, so as to form what is called “outlet channels” 18s and “inlet channels” 18e, respectively (see FIG. 4). What is then obtained is a “filter” block.
At the opposite ends of the outlet channels 18s and net channels 18e to the upstream 30s and downstream 30e plugs, respectively, the outlet channels 18s and the net channels 18e open to the outside via downstream openings, called “outlet openings 32s”, and via upstream openings, called “inlet openings 32e”, respectively, which extend over the downstream face 26s and upstream face 26e, respectively.
Thus, the inlet and outlet channels define net and outlet chambers 34e and 34s, respectively, each bounded by a lateral wall 22, a closure plug and an opening that opens to the outside. Two adjacent net and outlet channels are in fluid communication via their common lateral wall.
To manufacture an assembled filter body, the unitary filter blocks 11 are assembled together by bonding them by means of joints 12 made of a ceramic jointing cement interposed between their facing adjacent faces. The jointing cement generally consists of silica and/or silicon carbide and/or aluminum nitride. Preferably, the jointing cement is substantially impermeable to the exhaust gas to be filtered. The jointing cement may have a thermal conductivity of at least 0.1 W/m·K between 20° C. and 800° C. so as to limit the thermomechanical stresses. Typically, the average thickness of a joint 12 is between 0.3 and 4 mm.
The jointing cement may be applied over the entire area of a lateral face of a unitary filter block or over only part of this lateral face. In the latter base in particular, all the lateral faces of the unitary filter block cannot always be assembled indiscriminately to any lateral face of another unitary filter block. In other words, it may be necessary to identify one or more of the lateral faces of the unitary filter blocks so as to ensure that the lateral faces bonded together properly correspond. A mark may be printed for this purpose on the lateral faces of the unitary filter blocks.
The assembly thus formed may then be machined so as to obtain a shape matched to the can 5, for example for manufacturing a cylindrical filter body of circular cross section.
Generally, a peripheral coating 36, also called an “external coating” or “coating” made of a coating cement which is thermally insulating and impermeable to the exhaust gas, is applied to the lateral surface 38 of the monolithic or assembled filter body.
The assembled or monolithic filter body 3 may then be inserted into the can 5, a peripheral joint 40, impermeable to the exhaust gas, being placed between the lateral surface 38 of the filter body and the can 5.
The stream F of exhaust gas between the filter body 3 via the net openings of the net channels passes through the lateral filtering was of these channels before rejoining the outlet channels, and then escapes to the outside via the outlet openings.
After a certain period of use, the particulates, or “soot particles”, which have accumulated in the net channels of the filter body 3 increase the pressure drop due to the filter body 3 and thus impair the performance of the engine. For this reason, the filter body must be regularly regenerated, for example every 500 kilometers.
The regeneration or “declogging” operation consists in oxidizing the soot particles by heating them to a temperature enabling them to be burnt off.
During the regeneration phases, the temperature differs along the zones of the filter body 3 and does not vary uniformly. This is because the exhaust gas transports the thermal energy released by burning the soot to the downstream end. In addition, the soot is not deposited uniformly in the various channels, it accumulating for example preferentially in that zone of the filter body close to its longitudinal axis, also called the “core” of the filter body. The combustion zones are therefore not uniformly distributed within the filter body 3. The combustion of the soot therefore causes a temperature rise in the core of the filter body greater than that in the peripheral zones. Finally, the peripheral zones of the filter body 3 are cooled, through the metal can 5, by the surrounding air.
The inhomogeneity of the temperatures within the filter body 3 generates local stresses of large amplitude that may result in local ruptures or cracks. The filter body 3 must therefore be changed, the spent filter body preferably being recycled.
To increase the lifetime of the filter bodies, it is also possible to carry out thorough cleaning so as to clear the net channels of the residual ash. This cleaning operation, conventionally called “ash cleaning” is especially darned out for the filters intended for heavy duty vehicles. It conventionally involves removing the filter body from the exhaust line and using a cleaning apparatus external to the vehicle for extracting the residues from the filter.
Whether for an external regeneration or recycling operation, it is necessary for the upstream and downstream faces of the filter body to be able to be rapidly identified.
There is therefore a need for a honeycomb structure that facilitates this identification.